1. Field of the Invention
The present invention relates to a color encoder that is used in an image reproduction apparatus such as a video-CD or CD-G player so as to serve as an interface between such an apparatus and a television receiver.
2. Description of the Prior Art
A color encoder produces a television signal from R, G, and B signals it receives. The television signal includes a vertical return trace blanking interval between vertical scanning intervals. As shown in FIG. 4, a vertical return trace blanking interval lasts for a time interval of 21 H (1 H represents the duration of one horizontal scanning interval). Within one vertical return trace blanking interval, the first 9 H time interval consists of a vertical synchronizing signal interval T2 lasting for 3 H and containing six upward-rising pulses (cut-in pulses K), and two equalizing intervals T1 and T3, one immediately preceding the vertical synchronizing interval and the other immediately succeeding it, of which each contains six downward-dropping pulses (equalizing pulses Q). This first 9 H time interval of the vertical return trace blanking interval serves to stabilize the horizontal and vertical scanning of a television receiver. The intervals T1, T2, and T3 each last for a time interval of 3H. The remaining 12 H time interval of the vertical return trace blanking interval consists of horizontal scanning pulses HS and color bursts BST (a color synchronizing signal). To cope with such composition of the vertical return trace blanking interval, a color encoder blanks (suppresses) the R, G, and B signals it receives during the vertical return trace blanking interval.
In recent years, new image data transmission methods have been proposed to make effective use of the vertical return trace blanking interval. In such methods, various signals for testing and for transmitting character-based information (hereinafter referred to as closed-caption signals) are superposed on the R, G, and B signals during particular horizontal scanning intervals within the vertical return trace blanking interval, for example, from the 14th through 16th horizontal scanning intervals H.sub.14 to H.sub.16 and during the 21st horizontal scanning interval H.sub.21.
However, since a conventional color encoder as described above blanks the R, G, and B signals without fail during every vertical return trace blanking interval, it cannot deliver test signals or closed-caption signals to a television receiver even if such signals are transmitted during the vertical return trace blanking interval, and thus it does not allow the reproduction of those signals on the television receiver.
To solve this problem, a modified color encoder has been devised to allow the reproduction of closed-caption signals on a television receiver. Specifically, this color encoder is designed not to suppress the R, G, and B signals during particular horizontal scanning intervals within the vertical return trace blanking interval, for example, in the example noted above, from the 14th through 16th horizontal scanning intervals H.sub.14 to H.sub.16 and during the 21st horizontal scanning interval H.sub.21.
This type of color encoder, however, admits and processes the R, G, and B signals without fail during particular intervals within the vertical return trace blanking interval, and accordingly, even if the signals thus admitted include signals other than test or closed-caption signals, those signals are delivered to the television receiver and may cause malfunction thereof.